Conventionally, backlight illumination devices using a cold cathode fluorescent tube have been widely used in liquid crystal display devices with liquid crystal display panels. In recent years, attention has been focused on backlight illumination using three color light emitting diodes (LED devices) of red, green and blue lights for the reproduction of more clear and natural color tones, and the development thereof has been vigorously promoted.
A planar illumination device of the lateral light source type so-called edge light type is used as a backlight illumination device with a relatively small size, wherein light emitted from a light source is incident on a side surface (incident surface) of a light guide plate and light is emitted from one principal surface (light output surface) of the light guide plate for illumination.
The light guide plate adopted in this method uses a principal surface at a side opposite to the light output surface as a light reflecting surface and a dimming pattern is applied to either one of the principal surfaces of the light guide plate to make the output light uniform.
Known dimming patterns include the one formed by providing a plurality of random reflection areas on a reflecting surface of a transparent light guide plate by printing or the like in such a manner as to increase the density of the random reflection areas or a degree of scattering of the random reflection areas with distance from a light source (as disclosed in for example, Patent Documents 1, 2) and the one formed by providing recesses or projections in or on the reflecting surface of the light guide plate in such a manner as to successively increase the depths or the heights thereof with distance from the light source (as disclosed in for example, Patent Document 3).
A direct illumination device wherein cathode fluorescent tubes or LED devices are arranged in a planar manner is used for backlight illumination requiring a large size and a high luminance.
An increasing demand can be expected in the feature for liquid crystal display devices with thin and large screens, for use in for example wall mounted TVs, however, such direct illumination devices have such drawback in that it is more difficult to realize a thinner liquid crystal display device as compared to the case of edge light type illumination devices, and are less suited for thinner liquid crystal display devices. On the other hand, for the edge light type illumination devices, although it is difficult to ensure a sufficient luminance when adopting larger screens, it is possible to place them into practical applications by increasing the luminance of light sources.
In response, edge light type planar illumination devices using laser light sources which have higher luminance than LED devices and which are suited for higher outputs have been studied to realize thin liquid crystal display devices with large screens.
For the edge light type planar illumination device adopting laser light sources may be arranged, for example, as shown in FIGS. 9A and 9B. FIGS. 9A and 9B are a plan view and a side view schematically showing structures of a conventional planar illumination device adopting laser light sources.
In FIGS. 9A and 9B, diffusing light emitted from a high-output laser light source 10 is incident on a light guide plate 12 after being formed into luminous flux substantially parallel at least in the thickness-wise direction of the light guide plate 12 by a cylindrical lens 11.
The foregoing structure, wherein the light is incident on the light guide plate 12 from a direction substantially parallel to the thickness-wise direction of the light guide plate 12, has the following problem.
The laser light, incident on the light guide plate 12 from the direction substantially parallel to the light guide plate 12, propagates in vicinity of the light guide plate 12 as the substantially parallel light with its directivity. Thus, the amount of the light polarized by reflection or scattering is smallest at the light incident surface side of the light guide plate 12. On the other hand, the density of a dimming pattern 12a of the conventional light guide plate 12 is set to be sparse at the light incident surface side of the laser light from the laser light source 10 and to increase with distance from the laser light source 10. Accordingly, in the case of applying the dimming pattern 12a of the conventional light guide plate 12 to the laser light source 10, an effect of polarizing the light by reflection or scattering is further reduced in vicinity of the light incident surface of the light guide plate 12 and most of the luminous flux is totally reflected in vicinity of the light incident surface. The totally reflected luminous flux propagates to the inside of the light guide plate 12 without being emitted from the light guide plate 12. As a result, the amount of the output light in vicinity of the light incident surface decreases to make the luminance of the emitted light from the light guide plate 12 nonuniform.    Patent Document 1:    Japanese Unexamined Patent Publication No. H01-241590    Patent Document 2    Japanese Unexamined Patent Publication No. H06-130365    Patent Document 3:    Japanese Unexamined Patent Publication No. H06-230380